The present invention relates to an electro-optic passive display cell comprising two plates at least one of which, i.e. the front one, is transparent, and between which is imprisoned a layer of a mixture comprising among others a nematic liquid crystal.
The display cells having a liquid crystal of the twisted nematic type are generally thus realized, the two plates being transparent. Unfortunately, they need the presence of polarizers.
One has suggested to add dichroic molecules to the liquid crystal. This type of cells is disclosed in the publication of G. H. Heilmeier et al., Molecular Crystals and Liquid Crystals, 1969, Vol. 8, pp 293-304. If the liquid crystal has a positive dielectric anisotropy, the alignment being homogeneous planar on the two plates, the display appears in clear on a dark background, which is not favorable from the physiological point of view. A display in dark on a clear background can be obtained by using a liquid crystal having a negative dielectric anisotropy, the alignment being homeotropic on the two plates. For this type of liquid crystal, the absolute value of the dielectric anisotropy .vertline..DELTA..epsilon..vertline. is substantially lower than for the liquid crystals having a positive dielectric anisotropy which are generally used. Therefore, the control voltage is higher. One can place a polarizer in front of the cell to increase the contrast, but that diminishes the brilliancy of the background.
The solution disclosed by Taylor et al. in U.S. Pat. No. 3,833,287 realizes a cell having a good contrast and a high brilliancy without a polarizer. In this case, the display appears always in clear on a dark background. This solution is based on the fact that while adding an optically active compound to the liquid crystal, one induces a helicoidal structure the pitch p of which is a function of the concentration of the optically active component in the mixture, so that the dichroic molecules associated to the mixture absorb all the components of the light.
The purpose of the invention is to furnish a electro-optic passive display cell in which the display is effected in dark on a clear or transparent background while using a mixture of a liquid crystal having a positive dielectric anisotropy, an optically active component and dichroic molecules, such as is disclosed in the referenced U.S. Pat. No. 3,833,287.
The structure of the layer of the mixture is determined both by the type of alignment of the molecules on the surfaces of the plates and by the thickness of the said layer. If this thickness is sufficiently important, the structure is always helicoidal in the mass of the mixture, independently of the type of alignment on the surfaces.
For or in the presence of homeotropic alignment on each of the two plates, there exists a critical thickness of the layer, which depends on the pitch p of the helix and on the elastic constants of the mixture; under or less than this thickness the structure of the said layer is homeotropic in or through its whole thickness. This critical value is given by the formula: ##EQU1## where L.sub.c1 : critical thickness of the layer
p: pitch of the helicoidal structure of the mixture without stress PA1 k.sub.22 : elastic constant for a deformation of torsion of the mixture PA1 k.sub.33 : elastic constant for a deformation of flexion of the mixture.
This formula is drawn from an article of "Applied Physics Letters", Vol. 25, No 1, July 1, 1974, W. Greubel, pp. 5 to 7.
Thus, for a cell the alignment of which is homeotropic on the two faces and in which the thickness of the layer is lower or less than the critical value, the molecules are aligned perpendicularly to the plates in or through the whole thickness of the cell. On the other hand, for a cell in which the thickness of the layer is higher or greater than the critical value, the molecules are well aligned perpendicularly to the plates in the vicinity of the plates, but, in a median portion, the helicoidal structure will appear.